Lesions induced in DNA may be repaired by "error-proof" or "error-prone" repair mechanisms. Error-prone systems repair the lethal damage of the lesion, but in doing so may fail to restore the original nucleotide sequence, producing mutations. This error-prone process is called misrepair mutagenesis. Bacteriophage T4 has its own misrepair pathway, mediated by at least three genes, x, y, and w. Mutations in two of these genes, x and y, suppress mutations of gene 49, a gene involved in DNA packaging. This allows the selection of alleles of x and y. Amber and ts x and y mutants are being sought. These will allow the time course of mutagenesis to be examined. An rII reversion system to assay misrepair mutagenesis is being developed that will increase the sensitivity and allow measurements of mutagenesis along certain defined pathways. Finally the enzymology of T4 misrepair mutagenesis will be studied by genetic and biochemical approaches.